Synthesis gas for production of for instance methanol, dimethyl ether (DME) or liquid hydrocarbons via for instance Fischer-Tropsch synthesis, may be produced from carbon-containing feedstock such as natural gas, LPG, liquid hydrocarbons including heavy hydrocarbons, or solid feedstock such as coal. The carbon-containing feedstock is reacted with steam and/or air, enriched air, or oxygen at high temperature during steam reforming, autothermal reforming, catalytic partial oxidation or combinations thereof.
In the conventional steam reforming process natural gas or light hydrocarbons are reacted with steam in the presence of a catalyst based on nickel or noble metals. Temperatures at the reactor outlet of up to 950° C. are obtained. During autothermal reforming (ATR) or catalytic partial oxidation (CPO), natural gas or light hydrocarbons are reacted with steam and an oxidant (air, enriched air, or oxygen) in the presence of a catalyst based on nickel or noble metals. Temperatures up to 1100° C. are usually obtained at the outlet of the reactor. During non-catalytic partial oxidation (POX) of natural gas, light hydrocarbons, heavy hydrocarbons or solid feedstock such as coal (also referred to as gasification) is reacted with an oxidant (air, enriched air or oxygen) and outlet temperatures from the reactor of up to 1400° C. are obtained.
These processes are well known to those experienced in the art. A comprehensive description of the individual processes and relevant variations and combinations thereof is given by e.g. Aasberg-Petersen et al. Fischer-Tropsch Technology, Stud. Surf. Sci. Catal. 152 (2004) 258-405, edited by Steynberg, A. P. and Dry, M. E.
In processes based on steam reforming and/or autothermal reforming or catalytic partial oxidation the composition of the synthesis gas may be an equilibrium mixture of hydrogen, carbon monoxide, carbon dioxide, methane and steam established at the outlet temperature and pressure of the last catalytic reactor according to the reactions:Steam reforming: CH4+H2O═CO+3H2  (1)Water Gas Shift: CO+H2O═CO2+H2  (2)
In partial oxidation the equilibrium may be established at a temperature somewhat lower than the outlet temperature from the reactor. Hydrocarbons other than CH4 will generally be present in synthesis gas produced by any of the methods only in small or insignificant amounts. However, certain other components may also be present in trace amounts as impurities with possible detrimental effects in downstream processes, especially if the feedstock or the oxidant contains nitrogen.
Impurities of special interest are ammonia, hydrogen cyanide, formic acid, and sulphur compounds (abbreviated S-compounds), especially hydrogen sulphide (H2S) and carbonyl sulphide (COS). Ammonia, hydrogen cyanide, and formic acid will be present in amounts corresponding to establishment of equilibrium (at the same conditions as the equilibrium for reactions (1) and (2)) for the following reactions:3H2+N2=2NH3  (3)CO+NH3═HCN+H2O  (4)CO+H2O═HCOOH  (5)
The concentration of ammonia may be up to a few hundred vol ppm, whereas the concentration of hydrogen cyanide and formic acid will normally be less than 100 vol ppm.
In cases where the synthesis gas is produced by steam reforming, autothermal reforming or catalytic partial oxidation over a catalyst, all sulphur is usually removed from the feedstock, because it is a poison for the catalysts employed in these processes. In other cases sulphur is not completely removed before the autothermal reforming step or the catalytic partial oxidation step. In cases where the synthesis gas is produced by partial oxidation, sulphur is usually not removed from the feedstock, and the total concentration of sulphur compounds in the synthesis gas thus depends on the amount of sulphur (in any form) in the feedstock.
The ratio between hydrogen sulphide and carbonyl sulphide corresponds to establishment of equilibrium for the reaction:CO2+H2S═COS+H2O  (6)
The equilibrium is established at the same conditions as for reaction (1)-(5).
After leaving the reactor, where the synthesis gas is formed, the raw synthesis gas is cooled in one or more steps to a temperature where most of its content of water vapour condenses. The first cooling step can be used to produce steam followed by cooling in air and/or water cooling.
The synthesis gas is often rich in carbon monoxide, and this may induce risk of carbon formation on catalysts or metal dusting corrosion on the equipment in the cooling section. These risks are known to be reduced by the presence of sulphur compounds. Therefore, if the feedstock is treated before the conversion to synthesis gas by removal of sulphur, sulphur containing compounds may in certain cases be added before partial or full conversion of the feedstock or before the cooling of the synthesis gas in order to reduce the risk for carbon formation on catalysts or for metal dusting in the cooling section.
After cooling of the synthesis gas, condensate is separated, and the synthesis gas is sent to the section for synthesis of the final product e.g. methanol or dimethyl ether (DME) or hydrocarbons. The condensate will comprise dissolved gases including carbon oxides, most of the ammonia, and almost all of the formic acid. The pH of the condensate will typically be around 7.
Hydrogen cyanide and hydrogen sulphide will at this pH not be dissociated in the water, and they will, together with carbonyl sulphide and other non-dissociated gases, be distributed between gas and condensate according to the relevant vapour/liquid equilibria. The synthesis gas will thus, in addition to the main components hydrogen, carbon monoxide, carbon dioxide and methane, contain traces of ammonia, hydrogen cyanide, and sulphur compounds, abbreviated S-compounds. The condensate will contain the dissolved gases comprising ammonia, hydrogen cyanide, S-compounds and formic acid.
The content of ammonia, hydrogen cyanide, S-compounds and formic acid in both the synthesis gas and the condensate may cause problems in downstream process steps. In synthesis of methanol or DME, ammonia and hydrogen cyanide will be converted to methyl amines, which are undesired in the products and must be removed, e.g. by ion exchange. A more serious effect is seen in hydrocarbon synthesis by Fischer-Tropsch reactions, especially when catalysts based on Co are used, see e.g. U.S. Pat. No. 6,107,353. In such cases, ammonia and hydrogen cyanide may act as catalyst poisons by unfavourably affecting the activity and selectivity of the synthesis catalyst. S-compounds are strong catalyst poisons and cannot be tolerated at more than very low concentrations, in some cases below a total concentration of 60 ppb (Equipment Design and Cost Estimation for Small Modular Biomass Systems, Synthesis Gas Cleanup, and Oxygen Separation Equipment, National Renewable Energy Laboratory (NREL), Subcontract Report SR-510-39947, task 9, section 2.1.1, California, U.S.A. (May 2006), also available on http://www.nrel.gov/docs/fy06osti/39947.pdf) or lower. The presence of ammonia, hydrogen cyanide and sulphur compounds in the synthesis gas is thus undesirable. The content of ammonia depends strongly on the temperature in the condensate separator; it is highest when the temperature is relatively higher. However, the traces of ammonia are easily removed by washing with water. Hydrogen cyanide and the sulphur compounds in the synthesis gas are more difficult to remove since their solubility in water is limited at the prevailing conditions.
The condensate is most often purified by flashing and/or stripping with steam followed by final purification by ion exchange. A survey of various concepts for stripping of process condensate may be found in J. Madsen: Ammonia Plant Saf. 31 (1991) 227-240.
Hydrogen cyanide may be removed by flashing or low temperature steam stripping (with low pressure steam at 100-120° C.) together with other dissolved gases including ammonia and carbon dioxide in so-called overhead gases. However, hydrogen cyanide may cause undesirable corrosion in the equipment, even when this is made from stainless steel. If the stripping is done at higher temperature, e.g. by stripping with medium pressure steam at 230-250° C., hydrogen cyanide may be converted to formic acid by the following reactions:HCN+H2O═HCONH2  (7)HCONH2+H2O═HCOOH+NH3  (8)
Formic acid will not be removed by the stripping process. It must be removed by the final purification by ion exchange and constitutes a major part of the load on this process step and thus of the consumption of chemicals required for regeneration of the ion exchange resin.
It is thus evident that it is desirable to remove hydrogen cyanide from the wet synthesis gas before water vapour is condensed so that the content of hydrogen cyanide in both the dry synthesis gas and the process condensate is reduced. It is further evident that this removal or reduction of hydrogen cyanide is preferably done in such a way that the reactions (7) and (8) do not take place in the gas phase, leading to increased content of formic acid in the process condensate. Most preferable is a process which in addition to the removal of hydrogen cyanide from the synthesis gas also removes the formic acid formed in the synthesis gas generator by reaction (5). Such removal of formic acid can be effected by a process which in addition to the decomposition of hydrogen cyanide according to reaction (4), which is reversed at lower temperatures, also decomposes formic acid, e.g. according to the following reaction:HCOOH→CO2+H2  (9)
U.S. Pat. No. 4,521,387 discloses a process for purifying gases containing CO and/or CO2 by removing sulphur compounds, free unreacted oxygen, hydrogen cyanide, hydrogen chloride, mercury and other compounds. The gas to be purified is passed directly through a catalyst charge containing a Cu/ZnO catalyst prepared by the thermal decomposition of a mixed crystalline compound of zinc hydroxide carbonate. The removal of sulphur compounds, metal carbonyls and hydrogen cyanide by the catalyst is shown in process steps that either include passage through the catalyst alone or passage through active carbon followed by passage through the catalyst. There is no indication of the catalyst's ability to remove the other compounds mentioned.
It is also desirable that reactions such as methanation (the reverse of reaction (1)) or the shift reaction (reaction (2)) are not promoted.
These reactions are undesirable since they could change the overall composition of the synthesis gas in an undesirable way and, especially in the case of Fischer-Tropsch synthesis using catalysts based on Co, cause loss of production capacity. However, the potential problems caused by the presence of the sulphur compounds also need to be reduced. Moreover, the presence of sulphur compounds in the feed to the process will be detrimental to the performance of the process itself, since sulphur is a poison to the catalyst employed in the process. Therefore, it is desirable to remove the sulphur compounds from the synthesis gas before it is further processed for removal of hydrogen cyanide and formic acid and its derivatives.
It is an objective of the invention to provide a process, by which the content of sulphur compounds, hydrogen cyanide and formic acid and formic acid derivatives, is simultaneously reduced or removed from synthesis gas.